Make libs/exec fallback to sh if bash cannot be found (#1114)

## Changes

Falling back to `sh` is also what GitHub Actions do if `bash` is not
found in the path. It is possible `bash` is not available when running
from minimal Docker containers and we must not error out in this case.

See:
https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#jobsjob_idstepsshell.

This change renames `interpreter` -> `shell`.

## Tests

Unit tests pass.

libs/exec/exec.go

@@ -46,23 +46,23 @@ func (c *command) Stderr() io.ReadCloser {
 }
 
 type Executor struct {
-	interpreter interpreter
-	dir         string
+	shell shell
+	dir   string
 }
 
 func NewCommandExecutor(dir string) (*Executor, error) {
-	interpreter, err := findInterpreter()
+	shell, err := findShell()
 	if err != nil {
 		return nil, err
 	}
 	return &Executor{
-		interpreter: interpreter,
-		dir:         dir,
+		shell: shell,
+		dir:   dir,
 	}, nil
 }
 
 func (e *Executor) StartCommand(ctx context.Context, command string) (Command, error) {
-	ec, err := e.interpreter.prepare(command)
+	ec, err := e.shell.prepare(command)
 	if err != nil {
 		return nil, err
 	}

libs/exec/exec_test.go

@@ -2,8 +2,11 @@ package exec
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"io"
+	"os"
+	osexec "os/exec"
 	"runtime"
 	"sync"
 	"testing"
@@ -49,51 +52,63 @@ func TestExecutorWithInvalidCommandWithWindowsLikePath(t *testing.T) {
 	assert.Contains(t, string(out), "C:\\Program Files\\invalid-command.exe: No such file or directory")
 }
 
-func TestFindBashInterpreterNonWindows(t *testing.T) {
-	if runtime.GOOS == "windows" {
-		t.SkipNow()
+func testExecutorWithShell(t *testing.T, shell string) {
+	p, err := osexec.LookPath(shell)
+	if err != nil {
+		if errors.Is(err, osexec.ErrNotFound) {
+			switch runtime.GOOS {
+			case "windows":
+				if shell == "cmd" {
+					// We must find `cmd.exe` on Windows.
+					t.Fatal("cmd.exe not found")
+				}
+			default:
+				if shell == "bash" || shell == "sh" {
+					// We must find `bash` or `sh` on other operating systems.
+					t.Fatal("bash or sh not found")
+				}
+			}
+			t.Skipf("shell %s not found", shell)
+		}
+		t.Fatal(err)
 	}
 
-	interpreter, err := findBashInterpreter()
-	assert.NoError(t, err)
-	assert.NotEmpty(t, interpreter)
-
-	e, err := NewCommandExecutor(".")
-	assert.NoError(t, err)
-	e.interpreter = interpreter
+	// Create temporary directory with only the shell executable in the PATH.
+	tmpDir := t.TempDir()
+	t.Setenv("PATH", tmpDir)
+	if runtime.GOOS == "windows" {
+		os.Symlink(p, fmt.Sprintf("%s/%s.exe", tmpDir, shell))
+	} else {
+		os.Symlink(p, fmt.Sprintf("%s/%s", tmpDir, shell))
+	}
 
+	executor, err := NewCommandExecutor(".")
 	assert.NoError(t, err)
-	out, err := e.Exec(context.Background(), `echo "Hello from bash"`)
+	out, err := executor.Exec(context.Background(), "echo 'Hello from shell'")
 	assert.NoError(t, err)
-
-	assert.Equal(t, "Hello from bash\n", string(out))
+	assert.NotNil(t, out)
+	assert.Contains(t, string(out), "Hello from shell")
 }
 
-func TestFindCmdInterpreter(t *testing.T) {
-	if runtime.GOOS != "windows" {
-		t.SkipNow()
+func TestExecutorWithDifferentShells(t *testing.T) {
+	for _, shell := range []string{"bash", "sh", "cmd"} {
+		t.Run(shell, func(t *testing.T) {
+			testExecutorWithShell(t, shell)
+		})
 	}
+}
 
-	interpreter, err := findCmdInterpreter()
-	assert.NoError(t, err)
-	assert.NotEmpty(t, interpreter)
-
-	e, err := NewCommandExecutor(".")
-	assert.NoError(t, err)
-	e.interpreter = interpreter
-
-	assert.NoError(t, err)
-	out, err := e.Exec(context.Background(), `echo "Hello from cmd"`)
-	assert.NoError(t, err)
-
-	assert.Contains(t, string(out), "Hello from cmd")
+func TestExecutorNoShellFound(t *testing.T) {
+	t.Setenv("PATH", "")
+	_, err := NewCommandExecutor(".")
+	assert.ErrorContains(t, err, "no shell found")
 }
 
 func TestExecutorCleanupsTempFiles(t *testing.T) {
 	executor, err := NewCommandExecutor(".")
 	assert.NoError(t, err)
 
-	ec, err := executor.interpreter.prepare("echo 'Hello'")
+	ec, err := executor.shell.prepare("echo 'Hello'")
 	assert.NoError(t, err)
 
 	cmd, err := executor.start(context.Background(), ec)

libs/exec/shell.go

@@ -0,0 +1,54 @@
+package exec
+
+import (
+	"errors"
+	"io"
+	"os"
+)
+
+type shell interface {
+	prepare(string) (*execContext, error)
+}
+
+type execContext struct {
+	executable string
+	args       []string
+	scriptFile string
+}
+
+func findShell() (shell, error) {
+	for _, fn := range []func() (shell, error){
+		newBashShell,
+		newShShell,
+		newCmdShell,
+	} {
+		shell, err := fn()
+		if err != nil {
+			return nil, err
+		}
+
+		if shell != nil {
+			return shell, nil
+		}
+	}
+
+	return nil, errors.New("no shell found")
+}
+
+func createTempScript(command string, extension string) (string, error) {
+	file, err := os.CreateTemp(os.TempDir(), "cli-exec*"+extension)
+	if err != nil {
+		return "", err
+	}
+
+	defer file.Close()
+
+	_, err = io.WriteString(file, command)
+	if err != nil {
+		// Try to remove the file if we failed to write to it
+		os.Remove(file.Name())
+		return "", err
+	}
+
+	return file.Name(), nil
+}

libs/exec/shell_bash.go

@@ -0,0 +1,37 @@
+package exec
+
+import (
+	"errors"
+	osexec "os/exec"
+)
+
+type bashShell struct {
+	executable string
+}
+
+func (s bashShell) prepare(command string) (*execContext, error) {
+	filename, err := createTempScript(command, ".sh")
+	if err != nil {
+		return nil, err
+	}
+
+	return &execContext{
+		executable: s.executable,
+		args:       []string{"-e", filename},
+		scriptFile: filename,
+	}, nil
+}
+
+func newBashShell() (shell, error) {
+	out, err := osexec.LookPath("bash")
+	if err != nil && !errors.Is(err, osexec.ErrNotFound) {
+		return nil, err
+	}
+
+	// `bash` is not found, return early.
+	if out == "" {
+		return nil, nil
+	}
+
+	return &bashShell{executable: out}, nil
+}

libs/exec/shell_bash_test.go

@@ -0,0 +1,30 @@
+package exec
+
+import (
+	"runtime"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestBashFound(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.SkipNow()
+	}
+
+	shell, err := newBashShell()
+	assert.NoError(t, err)
+	assert.NotNil(t, shell)
+}
+
+func TestBashNotFound(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.SkipNow()
+	}
+
+	t.Setenv("PATH", "")
+
+	shell, err := newBashShell()
+	assert.NoError(t, err)
+	assert.Nil(t, shell)
+}